Meteorology: Weak polar vortex makes weather more predictable

Events in the stratosphere are making long-range weather in Northern Europe easier to forecast, researchers at LMU have discovered.

Weather is a chaotic system and predicting weather conditions several weeks in advance poses considerable challenges.

The accuracy of such long-range forecasts remains generally quite low.

Accordingly, even moderate improvements can prove valuable for various sectors.

For instance, farmers rely on these forecasts to determine optimal sowing and harvesting times, energy providers use them to anticipate fluctuations in renewable energy production, and public health officials use them to prepare for outbreaks of diseases such as malaria or dengue fever, which are correlated with specific weather conditions.

Researchers at LMU are now investigating a phenomenon that has its origin in the stratosphere, the layer of our atmosphere situated 15 to 50 kilometers above our heads.

"Previous work has shown that during Northern winter the state of the circulation in the polar stratosphere may provide useful information for improved long-range forecasts, especially for weather over the North Atlantic and Eurasia," explains Thomas Birner, Professor of Theoretical Meteorology at LMU.

In particular, when the polar vortex (a band of strong eastward circumpolar flow at stratospheric levels) strongly weakens or breaks down, the North Atlantic jetstream tends to shift southward and the likelihood of cold spells over Eurasia increases.

Such vortex breakdowns are relatively rare events that only happen approximately every other winter.

But its time has come round again: "One such event is currently unfolding with corresponding expected impacts on Eurasian weather in the coming weeks."

And now for the weather: cold, but less chaotic

In a study published recently in the journal Communications Earth & Environment, LMU meteorologists highlight an additional aspect of stratospheric influence on long-range weather forecasts: Weak polar vortex states, such as the one currently prevailing, are typically followed by reduced uncertainty of 3-5 week forecasts over Northern Europe.

The authors found that ensembles of forecasts show a reduced range of possible weather conditions by about 25%. Such ensembles are made up of a large number of individual forecasts, which typically diverge at longer forecasting periods.

After weak polar vortex events there is less spread among these forecasts over Northern Europe, making the weather more predictable.

"We attribute this reduced forecast uncertainty to the southward shift of the North Atlantic jetstream," says Jonas Spaeth, doctoral student at LMU's Meteorological Institute and lead author of the new study.

The associated southward shift of the tracks of winter storms, which are the main source of forecast uncertainty during this season, causes less storm activity and thereby reduced forecast uncertainty over Northern Europe.

Conversely, forecast uncertainty increases over Southern Europe.

Read more at Science Daily

The rich meteorology of Mars studied in detail from the Perseverance rover

Perseverance is a NASA autonomous vehicle that arrived at the Jezero Crater (the bed of an ancient, now dried-up lake on Mars) on 18 February 2021. The rover is equipped with seven novel, complex scientific instruments dedicated to exploring the planet's surface in search of signs of possible past life, collecting and depositing samples to be brought back to Earth, testing new technologies for use in human exploration, and studying the planet's atmosphere in detail. With regard to the aim of studying the atmosphere, the MEDA (Mars Environmental Dynamics Analyzer) instrument has been obtaining novel results. MEDA's lead researcher is José Antonio Rodríguez-Manfredi of the Centre for Astrobiology (CAB) in Madrid, and it has had the participation of a team from the UPV/EHU's Planetary Sciences Research Group. The instrument comprises a set of sensors that measure temperature, pressure, wind, humidity and properties of the dust that is always present in suspension in the Mars atmosphere.

Perseverance has now completed its investigation of the atmosphere throughout the first Martian year (which lasts approximately two Earth years). A preview of the results, which appears on the cover, is published today in the January issue of the journal Nature Geoscience. Specifically, the UPV/EHU team, formed by Agustín Sánchez-Lavega, Ricardo Hueso, Teresa del Río-Gaztelurrutia and the PhD student Asier Munguira, has led the study of the seasonal and daily cycles of temperature and pressure, as well as their significant variations on other time scales resulting from very different processes.

Throughout the seasons, the average air temperature at the Jezero Crater, located near the planet's equator, is around minus 55 degrees Celsius, but varies greatly between day and night, with typical differences of around 50 to 60 degrees. In the middle of the day, the heating of the surface generates turbulent movements in the air as a result of the rise and fall of air masses (convection) which cease in the evening, when the air settles.

Pressure sensors, on the other hand, show in detail the seasonal change of the tenuous Martian atmosphere produced by the melting and freezing of atmospheric carbon dioxide at the polar caps, as well as by a complex, variable daily cycle, modulated by thermal tides in the atmosphere. "The pressure and temperature of the Mars atmosphere oscillate with periods of the Martian solar day (somewhat longer than the Earth's, it averages at 24 hrs 39.5 min) and with their submultiples, following the daily cycle of sunshine greatly influenced by the amount of dust and the presence of clouds in the atmosphere," says Agustín Sánchez-Lavega, professor at the Faculty of Engineering -- Bilbao (EIB) and co-researcher on the Mars 2020 mission.

Both sensors are also detecting dynamic phenomena in the atmosphere that occur in the vicinity of the rover, for example, those produced by the passage of whirlwinds known as "dust devils" because of the dust they sometimes kick up, or the generation of gravity waves whose origin is not yet well understood. "The dust devils are more abundant at Jezero than elsewhere on Mars, and can be very large, forming whirlwinds more than 100 metres in diameter. With MEDA we have been able to characterise not only their general aspects (size and abundance) but also to unravel how these whirlwinds function," says Ricardo Hueso, lecturer at the Faculty of Engineering -- Bilbao (EIB).

MEDA has also detected the presence of storms thousands of kilometres away, very similar in origin to terrestrial storms, as shown by the images from orbiting satellites, and which move along the edge of the north polar cap, formed by the deposition of carbonic snow.

Within the rich variety of phenomena studied, MEDA has been able to characterise in detail the changes that have taken place in the atmosphere by one of the dreaded dust storms, such as the one that developed in early January 2022. Its passage over the rover led to abrupt changes in temperature and pressure accompanied by strong gusts of wind, which kicked up dust and hit the instruments, damaging one of the wind sensors.

Read more at Science Daily